As a light emitting device of current type, organic light-emitting diodes (OLEDs) have been increasingly applied to active matrix OLED of high performance. Conventional passive matrix OLED requires a shorter driving time for a single pixel with increase of display size, and thus needs to increase transient current and increase power consumption. In the meanwhile, application of large current may cause a voltage drop on lines of nanometer ITOs (Indium Tin Oxides) to be excessively large and cause the operational voltage of the OLED to be excessively high, and hence the efficiency thereof is decreased. However, active matrix OLED (AMOLED) can address these issues fairly well through inputting OLED current by switching transistors performing progressive scanning.
In design of the pixel circuit of the AMOLED, the issue that is mainly focused on is the nonuniformity of brightness of the OLED devices driven by the respective AMOLED pixel driving units.
First of all, AMOLED employs thin film transistors (TFTs) to construct the pixel driving unit so as to provide corresponding driving current for the light emitting devices. As known, the Low Temperature Poly-silicon (LTPS) TFTs or oxide TFTs are mostly used. As compared with a general A-Si TFTs, the LTPS TFTs and the oxide TFTs have characteristics of higher mobility and stability, and more suitable for the application of AMOLED display. However, due to limitation of crystallization process, LTPS TFTs fabricated on a glass substrate with large area often have a nonuniformity in terms of electrical parameters such as threshold voltage, mobility and the like, and this nonuniformity will be converted into difference in the driving current of the OLED devices and difference in brightness and can be perceived by human's eye, that is, a phenomenon of mura. Although the oxide TFTs is pretty good in terms of the uniformity of the process, similar to the A-Si TFTs, in case of being applied a voltage for a long time and high temperature, a drift will occur in the threshold voltage thereof, and since the display pictures are different, the drift amounts of the threshold values of the respective parts of the TFTs of the panel will be different, which will cause a difference in brightness display; since this difference is associated with a previous displayed images, it is generally presented as a phenomenon of image sticking.
Since the light emitting device of the OLED is a current driven device, in the pixel driving unit for driving the light emitting device to emit light, the characteristic of the threshold value of the driving transistor thereof has a great impact on the driving current and the final displayed brightness. Drift will occur in the threshold value of the driving transistor when the driving transistor is subjected to the voltage stress and illumination, and this drift in threshold value will be embodied as nonuniformity in brightness in terms of display effect.
In addition, in the pixel circuit of the known AMOLED, in order to eliminate the impact due to the threshold voltage of the driving transistor, the structure of the pixel circuit will be generally designed to be relatively complex, and this will directly lead to decrease in yield rate of pixel circuit of the AMOLED in the manufacture.
Therefore, in order to address the above issues, the present disclosure provides a pixel driving unit, a driving method thereof, as well as a pixel circuit.